For communication in cellular communication systems it is important to control the transport data format and user to be scheduled in order to optimize the throughput in the network.
In many cellular communication systems the mobile terminals report a perceived channel quality to the base stations. This channel quality is used by the base station for scheduling, for determining the amount of data that can be transmitted to a mobile terminal at a given time and, possibly, for other purposes as well.
For example, according to the 3GPP standard for WCDMA High-speed downlink packet access (HSDPA), the mobile terminal reports to the radio base station a channel quality indicator (CQI), which reflects the channel quality perceived by the mobile terminal. The CQI is an information element contained in the associated uplink signalling. The CQI reported by the mobile terminal reflects the receiver performance capability of the UE and is based on measurements of the CPICH and is reported at a predefined rate and averaging interval defined by the network.
The CQI is reported as an integer between 0 and 30, where 0 indicates that the mobile terminal is not ready to be scheduled and that no transmission should be made to the mobile terminal. Based on, among other things, the CQI, the base station will perform scheduling, that is, select the mobile terminal to transmit to, and also select the transport block format to use when transmitting to this mobile terminal, that is, the amount of data to be transmitted.
The 3G standard specifies that the mobile terminal should report a CQI aiming for a Hybrid Automatic Repeat Request (HARQ) Block Error Rate (BLER) of 10%, meaning that the mobile terminal will on average not be able to decode 10% of the transmissions.
The 3GPP standard also specifies requirements on the CQI reporting accuracy for the mobile terminal but it is expected that the actual reporting accuracy will vary depending on mobile terminal model and make. If the mobile terminal reports an incorrect CQI, the base station will base its scheduling and selection of transport block format on erroneous data resulting in lower throughput because of a less efficient use of the High-speed downlink shared channel (HS-DSCH). In particular, some mobile terminals tend to overestimate their receiving quality. These terminals will be scheduled for transmission more often than if their CQI had been correct. Also, the transmission will not be adapted to the terminal's real ability to receive data, so that a large number of blocks will not be received correctly. These blocks will have to be retransmitted, resulting in a lower throughput. In the worst case the Block error rate (BLER) can be up to 100%. Conversely, mobile terminals that underestimate their capability by sending a too low CQI, will not be scheduled as often as they should and will exhibit a BLER lower than 10% due to a too conservative TB selection by the radio network.
Thus, to optimize the traffic in the network from an overall point of view, the network will need to adjust the CQI reported by the mobile terminal, if it is not correctly reported by the terminal itself.
One known algorithm for adjusting the CQI is based on the ACK and NACK (acknowledgement and not acknowledgement) messages received from the mobile terminal. In this algorithm, if a NACK message is received, indicating that a transmission of data was not received correctly in the mobile terminal, the CQI is decremented while for each ACK message the CQI is incremented by the same or a different value.
The WCDMA standard specifies a number of parameters that may be used to control how often a mobile terminal reports its CQI.